1. Field of the Invention
The present invention relates to the field of powder metallurgy. More particularly, the present invention relates to methods which involve blending a relatively fine metal powder with a relatively coarse prealloyed metal powder to produce a mixture that has a widened sintering temperature window compared to that of the relatively coarse prealloyed metal powder. The present invention also relates to articles made from such powder mixtures.
2. Description of the Related Art
The science and industry of making and using powder metals is referred to as powder metallurgy. Powder metal compositions include elemental metals as well as metal alloys and compounds. A wide variety of processes are used to make powder metals, for example, chemical or electrolytic precipitation, partial vaporization of metal containing compounds, and the solidification of liquid metal droplets atomized from molten metal streams. The shapes of metal powder particles are influenced by the powder making method and range from spherical to irregular shapes. Powder metals particles range in size from submicron to hundreds of microns. Particle size is measured as a diameter for spherical powders or as an effective diameter for non-spherical powders.
Various techniques are employed to consolidate powder metals particles to form useful metal articles through the use of applied pressure and/or elevated temperatures. The powder metal to be consolidated is typically formed into a shape at room temperature and held in place through the use at least one restraining mechanism such as container walls, a fugitive binder material, or mechanical interlocking caused by pressing the powder metal particles together with high pressure in a die press. Examples of specific forming processes include powder containerization, solid free-forming layer-wise buildup techniques (for example, three-dimensional printing (3DP) and selective laser sintering (SLS)), metal injection molding (MIM), and metal powder die pressing. The term xe2x80x9cgreen articlexe2x80x9d is used herein to refer to the shaped powder metal article produced by this stage of the consolidation process. The green article is then heated to one or more elevated temperatures at which atomic diffusion and surface tension mechanisms become active to consolidate the powder metal by sintering. The term xe2x80x9csintered articlexe2x80x9d is used herein to refer to the consolidated powder metal article produced by this stage of the consolidation process. Although sintering may occur to some extent over a range of temperatures as the green article is being heated, the peak temperature to which the green article is heated is what is usually referred to as the xe2x80x9csintering temperature.xe2x80x9d Generally, the green article is held for a period of time ranging from a few minutes to a few hours at the sintering temperature, the length of time depending upon a variety of process and metallurgical system-related factors.
The heating of the green article is done in a controlled atmosphere or vacuum so as to protect the powder metal from undesired reactions with atmospheric constituents. The heating is also controlled so as to eliminate any fugitive binders from the green article. The consolidation of the green article into a sintered article is typically done at about atmospheric pressure or under vacuum. Some specialized techniques, however, such as hot isostatic pressing, hot uniaxial pressing, and hot extrusion, apply a pressure to the green article while it is hot to aid in the consolidation. In some processes, for example, in some embodiments of 3DP and SLS, consolidation is achieved through an infiltration process by wicking a liquid metal into the pores of the green article from a source external to the green article.
As the consolidation of the powder metal proceeds from green article to sintered article, the density of the article increases as some or all of its porosity is eliminated. Density, in this application, may be defined as xe2x80x9cabsolute density,xe2x80x9d which is the article""s mass per unit volume. Absolute density is expressed in terms such as grams per cubic centimeter. Density is also defined as xe2x80x9crelative density,xe2x80x9d which is the ratio of the absolute density of a powder metal article to the density which that article would have if it contained no porosity. Relative density is expressed in terms of a percentage, with a highly porous article having a low relative density and an article having no porosity having a relative density of 100%. The relative density of a green article depends on many factors and is sensitive to the method by which the green article was formed. Green article densities are generally in the range of about 50-90%. The relative densities of sintered articles also depend on a variety of factors, including parameters of the sintering process. The sintered article relative densities typically are in the range of 75-95%. For applications in which the mechanical strength of a sintered article is of importance, high relative densities are generally desired. The increase in the relative density from the green article stage to the sintered article stage is referred to herein as xe2x80x9cdensification.xe2x80x9d
Densification may proceed by xe2x80x9csolid state sintering,xe2x80x9d which is a term that describes the phenomena by which solid particles become joined together at contact points through the diffusion of atoms between the contacting particles. The number of point contacts for a given volume of powder and the ratio of surface area to particle volume increase as metal powder particle size decreases, which results in finer powder metals solid state sintering more readily and at lower temperatures than do larger powder metal particles.
Densification of a green article during sintering can be enhanced by the presence of a liquid phase within the green article. The enhancement occurs because of the relatively high atomic diffusion rates through a liquid as compared to a solid and because of the effect that the surface tension of the liquid has in drawing the solid particles together. The sintering that results from the presence of the liquid phase is identified as xe2x80x9cliquid phase sintering.xe2x80x9d In some powder metallurgical systems, the powder metal in the green article comprises a minor volume fraction of a relatively low temperature melting powder metal and a high volume fraction of a second type of powder metal which remains solid at the sintering temperature. For example, a tungsten carbide-cobalt green article may contain a low volume fraction cobalt powder, which is liquid at the sintering temperature, and a high volume fraction tungsten carbide powder, which remains solid at the sintering temperature.
An important variant of liquid phase sintering is supersolidus liquid phase sintering. Supersolidus liquid phase sintering is possible if a prealloyed powder metal passes into a solid-plus-liquid phase state upon heating. Referring to FIG. 1, which depicts a portion of an idealized temperature-composition equilibrium phase diagram 1 for an alloy system consisting of metal Y and metal Z, the horizontal axis 2 relates to composition with the left hand end 4 of horizontal axis 2 representing pure metal Y. The weight percentage of metal Z in the alloy composition increases linearly to the right along the horizontal axis 2. The vertical axis 6 relates to temperature, which increases in the upward direction. The phase diagram 1 contains two phase boundary lines, liquidus line 8 and solidus line 10, which divide the illustrated portion of phase diagram 1 into three phase regions: a liquid phase region 12 above liquidus line 8; a solid-plus-liquid phase region 14 between liquidus line 8 and solidus line 10; and a solid phase region 16 below solidus line 10. A pure metal, such as metal Y, upon heating from a temperature at which it is a solid, remains a solid until it reaches its melting point temperature, Tm 18, at which it melts, and is a liquid at temperatures above Tm 18. In contrast, a Y-Z alloy of composition X 20, upon heating from the solid phase region 16 along dotted line 22 remains completely solid only until it crosses the solidus line 10 at its solidus temperature Ts 24 and enters the solid-plus-liquid phase region 14. In this region, the alloy exists in part as a solid and in part as a liquid. Upon further heating, the solid fraction decreases and the liquid fraction increases until the temperature crosses the liquidus line 8 at the alloy""s liquidus temperature T1 26 into liquid phase region 12, wherein the alloy exists as all liquid. Upon cooling, the process is reversed, with the liquid transforming upon crossing the liquidus line 8 into a liquid-plus-solid slush with an ever decreasing amount of liquid until the solidus line 10 is crossed and the alloy once again becomes all solid.
xe2x80x9cSupersolidus liquid phase sintering,xe2x80x9d as used herein, refers to liquid phase sintering that occurs at a temperature that is between the solidus temperatures and the liquidus temperature of the particular alloy composition. Supersolidus liquid phase sintering takes advantage of the two phase, solid-plus-liquid phase region 14 as a means of providing a liquid phase for liquid phase sintering. For example, if supersolidus liquid phase sintering is done at temperature TS 28, a Y-Z alloy of composition X 20 is in the solid-plus-liquid phase region 14 and consists partly of a solid of composition Xs 30 and partly of a liquid of composition X1 32. The fraction of liquid present is equal to the length of the sintering temperature dotted line 34 that is between its intersection point 38 with the solidus line 10 and its intersection point 36 with the composition X dotted line 22 divided by the length of the sintering temperature dotted line 34 that is between its intersection point 38 with the solidus line 10 and its intersection point 40 with the liquidus line 8. A prealloyed metal powder is amenable to supersolidus liquid phase sintering if a sintering temperature exists for the prealloyed metal powder at which the prealloyed metal powder densifies by supersolidus liquid phase sintering without slumping. Slumping refers to a noticeable amount of gravity-induced distortion of a green article occurring during liquid phase sintering that causes the dimensions of the resulting sintered article to be outside of their respective dimensional tolerance ranges. However, in micro-gravity conditions, slumping refers to such distortions which are surface tension-induced, rather than gravity-induced.
The supersolidus liquid phase sintering of a powder metal is shown diagramatically in FIGS. 2A-C. Referring to FIG. 2A, a small portion 42 of a green article is shown at great magnification before heating. Small portion 42 includes powder metal particles 44 of a Y-Z alloy corresponding to the composition X 20 discussed above. It also includes a pore 46, which is indicated by the hatch area between the powder metal particles 44. Powder metal particles 44 each contain grains 48.
Referring to FIG. 2B, the temperature of the small portion 42 of the green article has been raised to sintering temperature TS 28 within the solid-plus-liquid phase region 14 of the phase diagram 1 that is shown in FIG. 1. A liquid phase 50 has formed between and around the grains 48 of the powder metal particles 44. The liquid phase 50 has begun drawing the powder metal particles 44 closer together, shrinking pore 46. Comparison with FIG. 2A shows that the grains 48 have changed shape as the liquid 50 forms and dissolution and reprecipitation processes occur.
Referring to FIG. 2C, the temperature of the small portion 42 of the green article is still at sintering temperature TS 28, but sufficient time has elapsed for the supersolidus liquid phase sintering to have progressed to the point where the pore 46 has been eliminated. Some of the grains 48 have become rearranged and changed in size and shape from their initial state. Although the amount of liquid phase 50 is the same as it was in FIG. 2B, it too has become redistributed. As a result of the sintering, the green article has densified.
The volume fraction of liquid phase that is present during any type of liquid phase sintering, including supersolidus liquid phase sintering, is very important. An insufficient amount of liquid phase may be ineffective in achieving the desired level of densification. Alternatively, an excess of liquid phase may result in slumping of the article. During liquid phase sintering, when most or all of the remaining solid grains or powder particles are surrounded by a liquid phase, the shape of the green article is maintained by surface tension forces which give a high viscosity to the liquid-solid combination. When the volume fraction of liquid phase is below a threshold level, this viscosity is sufficiently high to permit the sintering green article to retain its shape at the sintering temperature long enough for the densification to occur to a desired point. Above the threshold level, gravity-induced distortion exceeds a tolerable level before the desired level of densification is achieved. The maximum amount of liquid phase that can be tolerated during liquid phase sintering varies complexly and widely and must be determined empirically.
Maintaining the article at the sintering temperature too long will also result in slumping as gravity-induced viscous flow proceeds at a slow, but definite rate, even when the amount of liquid present at the sintering temperature is such that the viscosity is high. Additionally, time-related slumping may occur as coarsening of the solid phase grains or particles causes a net decrease in their surface area that is in contact with the liquid and a corresponding increase in the thickness of the liquid layer between the grains, thereby decreasing viscosity.
Supersolidus liquid phase sintering is substantially affected by the rate of change of the volume fraction of liquid phase with respect to temperature near the sintering temperature for a particular alloy. If the volume of liquid phase increases rapidly with temperatures, the alloy is considered to be very sensitive to the sintering temperature. In some cases, the furnace temperature deviation about a set-point temperature, which in industrial furnaces can be on the order of tens of degrees Celsius, can exceed the temperature range in which a proper range of liquid phase fraction amounts are present in the alloy. A temperature deviation to the low end of the temperature furnace temperature deviation range could produce insufficient densification whereas a deviation to the high end of the temperature range could result in slumping. High precision temperature control usually comes at the cost of lower through-put capacities and higher equipment prices.
Additionally, temperatures may vary from location to location within the working zone a furnace. Among the relevant factors are a location""s proximity to heating elements, load and fixture related shielding from radiative heating sources, and variations in gas flow patterns. Temperature variations during processing also occur within a green article as the outside of the article heats up before its interior. Such intra-article temperature variations are affected by the rate at which the furnace temperature is ramped up to the sintering temperature. For example, rapid ramping rates may cause large temperature differences between the outside and inside of a green article, whereas more moderate ramping rates allow time for better temperature equalization within the green article.
Any of the aforementioned process-related temperature variation factors may make it difficult or impracticable to sinter a particular green article. In some cases, although supersolidus liquid phase sintering of a particular green article is practicable, the processing-related temperature variations make it necessary to use a lower sintering temperature within the solidus/liquidus temperature range and require a correspondingly longer sintering time, in order to avoid slumping. This also has the disadvantageous effect of lowering the throughput capacity of the furnace.
What is needed in the art is a process that will widen the window of sintering temperatures within which a green article can be sintered to an acceptable density. Widening the sintering temperature window would make the sintering of the green article less sensitive to process-related temperature variations. This could translate into the ability to sinter the green articles in less expensive furnaces and at higher throughput rates.
The inventors have discovered that a green article comprising an A-B powder mixture may be sintered without slumping over a widened temperature range. Such an A-B powder mixture is made by mixing a minor volume fraction of a relatively fine metal powder A, which has a melting or solidus temperature that effectively exceeds the sintering temperature at which the powder mixture containing that powder is sintered, with a complementary major volume fraction of a relatively coarse prealloyed metal powder B, which is an alloy amenable to supersolidus liquid phase sintering. A green article comprising the A-B powder mixture may be sintered without slumping into a solid article at a sintering temperature that is within a wider temperature range than can a corresponding article which does not contain a volume fraction of the relatively fine metal powder A. xe2x80x9cRelatively finexe2x80x9d and xe2x80x9crelatively coarsexe2x80x9d signify that the mean particle sizes of the selected metal powders A and B are related by a ratio of about 1:5 or higher, that is, that the mean particle size of metal powder B is at least about 5 times larger than the mean particle size of metal powder A. Metal powders A and B may be of any shape.
The term xe2x80x9cvolume fractionxe2x80x9d for a given powder refers to the portion of the occupied volume of a powder mixture which is actually occupied by that particular powder. For example, in a powder mixture having a 100 cc occupied volume of which 30 cc is occupied by powder A and 70 cc is occupied by powder B, the volume fraction of powder A is 30% and the volume fraction of powder B is 70%. The volume of any fugitive or reactive additives that may be added to a powder mixture of identified components is not considered in determining the volume fraction of the identified components. Thus, an addition of 5 cc of a fugitive additive, such as a polymer binder, or of a reactive additive, such as carbon, to an A-B powder mixture consisting of 30 cc of powder A and 70 cc of powder B does not influence the determination that the volume fraction of powder A in the mixture is 30% and the volume fraction of powder B is 70%.
A green article xe2x80x9ccan be sintered without slumpingxe2x80x9d if no slumping occurs when the green article is sintered to an achievable desired relative density in a reasonable time. Prolonged exposure to sintering temperatures for unreasonably long times can cause slumping due to time-related effects. Also, those skilled in the art will recognize that there is a limit to the relative density that is achievable by sintering that places an upper limit on the relative density that can reasonably be achieved.
Green articles made from such A-B metal powder mixtures undergo substantial densification to a given relative density at lower sintering temperatures than do corresponding green articles of relatively coarse prealloyed metal powder B which do not contain a volume fraction of the relatively fine metal powder A. Moreover, green articles of such powder mixtures may be sintered at measurably higher temperatures within the solidus/liquidus temperature range of relatively coarse prealloyed metal powder B without slumping than can corresponding green articles of relatively coarse prealloyed metal powder B that do not contain a volume fraction of the relatively fine metal powder A. Thus, the sintering temperature window or range of a green article is effectively widened, thus making it less susceptible to the deleterious effects of temperature variations experienced in processing furnaces.
The relatively fine metal powder A particles occupy the interstices between the relatively coarse prealloyed metal powder B particles in a green article which comprises such an A-B powder mixture. Such particle packing in the powder mixture increases the relative density of the green article. The relatively coarse prealloyed metal powder B is difficult to sinter in the solid state, whereas the relatively fine metal powder A densifies relatively readily by solid state sintering. The benefit of using the fine powders is that, upon heating, solid state sintering of the relatively fine metal powder A densifies the green article to a higher density at a lower temperature in comparison to a corresponding green article of only the relatively coarse prealloyed metal powder B. Further increasing the sintering temperature above the solidus temperature, but below the liquidus temperature, of the relatively coarse prealloyed metal powder B softens the relatively coarse prealloyed metal powder B particles via a liquid phase that forms within those particles, penetrating their grain boundaries. The liquid phase formation also causes the fast densification that is typical of supersolidus liquid phase sintering. The combination of the solid state sintering contributions of the relatively fine metal powder A and the supersolidus liquid phase sintering contributions of the relatively coarse prealloyed metal powder B results in the blended powder mixture sintering by what may be defined as solid-supersolidus liquid phase sintering. Moreover, the volume fraction of relatively fine metal powder A dimensionally stabilizes the green article during this solid-supersolidus liquid phase sintering such that higher sintering temperatures can be employed without slumping in comparison to a corresponding green article of only relatively coarse prealloyed metal powder B.
It is to be understood that the present invention is not limited to methods and green articles comprising bimodal powder distributions. Higher-level poly-modal powder distributions are also contemplated, for example, trimodal distributions. Such distributions contain a major volume fraction of a relatively coarse prealloyed metal powder B, which is amenable to supersolidus liquid phase sintering, and a complementary minor volume fraction of relatively fine metal A consisting of sub-fractions such as sub-fractions A1, A2 and A3. Each of the relatively fine metal powder sub-fractions A1, A2, and A3 has a melting temperature or solidus temperature that exceeds the maximum sintering temperature at which the A-B powder mixture may be sintered without slumping. The mean particle sizes of metal powders A1 and B are related by a ratio of about 1:5 or higher and the mean particle sizes of each successive pair-wise combinations of relatively fine metal powders, for example, A1 and A2, A2 and A3, are related by a ratio of about 1:5 or higher. These size ratios allow the finer powders to nest within the interstices of the coarser powders.
It is also to be understood that in all cases, the relatively fine metal powder A must remain essentially solid at the sintering temperature. xe2x80x9cEssentially solidxe2x80x9d identifies that, on the average, the relatively fine metal powder A particles must retain sufficient structural integrity and physical size at the sintering temperature to act as physical barriers to the movement of the relatively coarse prealloyed metal powder B particles or grains thereof. Thus, some dissolution of metal powder A particles at or below the sintering temperature is permissible, as is the formation of a small amount of internal liquid within metal powder A particles. It is also permissible for the metal powder A particles to react with the metal powder B particles, even to form a small amount of liquid, so long as the structural integrity and size criteria are met. In embodiments using higher-level poly-modal distributions, each of the relatively fine metal powder sub-fractions must be essentially solid at the sintering temperature.
It is also to be understood that the relatively fine metal powder A may consist one or more elemental metals or alloys. For example, the powder volume fraction identified as relatively fine metal powder A may be made up a first volume sub-fraction of metal C and a second volume sub-fraction of metal D. In embodiments where higher-level poly-modal distributions are employed, each of the relatively fine metal powders sub-fractions, for example A1, A2 and A3, may consist of one or more elemental metals or alloys which may be the same or different from those in the other volume sub-fractions. The relatively coarse prealloyed powder B may also consist of one or more alloys.
Green articles comprising a powder metal mixture having a minor volume fraction of a relatively fine metal powder A and a complementary major volume fraction of a relatively coarse prealloyed metal powder B are also contemplated. In these embodiments, the relatively fine metal powder A is an elemental metal or alloy whose melting temperature or solidus temperature is higher than the highest sintering temperature at which the A-B powder mixture can be sintered without slumping and the coarse prealloyed metal powder B is an alloy that is amenable to supersolidus liquid phase sintering.
Methods of producing a green article having an enhanced sintering temperature range are also contemplated. Such a method includes the steps of mixing together a minor volume fraction of a relatively fine metal powder A and complementary major volume fraction of a relatively coarse prealloyed metal powder B to produce an A-B metal powder mixture, and forming a green article from said A-B metal powder mixture, wherein the relatively fine metal powder A is a metal or alloy whose melting temperature or solidus temperature is higher than the highest sintering temperature at which the A-B powder mixture can be sintered without slumping, and wherein the coarse prealloyed metal powder B is an alloy that is amenable to supersolidus liquid phase sintering.
Methods of densifying a green article are also contemplated. Such a method includes the steps of mixing together a minor volume fraction of a relatively fine metal powder A and complementary major volume fraction of a relatively coarse prealloyed powder B to produce an A-B metal powder mixture, forming a green article from said A-B metal powder mixture, and heating the green article to a sintering temperature below the liquidus temperature of the relatively coarse prealloyed metal powder B to densify the green article by sintering, wherein the relatively fine metal powder A is an elemental metal or alloy whose melting temperature or solidus temperature is higher than the highest sintering temperature at which the A-B powder mixture can be sintered without slumping, and wherein the relatively coarse prealloyed metal powder B is a metal alloy that is amenable to supersolidus liquid phase sintering. Where the relatively coarse prealloyed metal powder B consists of more than one alloy, the sintering temperature is a temperature that is lower than the liquidus temperature of each of the various relatively coarse prealloyed metal powder B alloys. In this method, the sintering temperature may, but need not, exceed the solidus temperature of the relatively coarse prealloyed metal powder B, or, where the relatively coarse prealloyed metal powder B consists of more than one alloy, the sintering temperature may, but need not, exceed the solidus temperature of any of those alloys.
Also contemplated are methods of solid-supersolidus liquid phase sintering a green article. Such a method comprises the steps of mixing together a minor volume fraction of a relatively fine metal powder A and a complementary major volume fraction of a relatively coarse prealloyed powder B to produce an A-B metal powder mixture, forming a green article from said A-B metal powder mixture, and heating the green article to a sintering temperature between the solidus and liquidus temperatures of the relatively coarse prealloyed metal powder B, wherein the relatively fine metal powder A is a metal or alloy whose melting temperature or solidus temperature is higher than the sintering temperature at which the A-B powder mixture can be sintered without slumping, and wherein the coarse prealloyed metal powder B is an alloy that is amenable to supersolidus liquid phase sintering. Where the coarse prealloyed metal powder B consists of more than one alloy, the sintering temperature is a temperature that exceeds the solidus temperature of each of the various relatively coarse prealloyed metal powder B alloys and is lower than the liquidus temperature of each of the various relatively coarse prealloyed metal powder B alloys.
It is to be specifically understood that embodiments related to corresponding green articles and methods involving higher-level poly-modal powder distributions are also contemplated.
Other features and advantages inherent in the subject matter disclosed and claimed will become apparent to those skilled in the art from the following detailed description of presently preferred embodiments thereof and to the appended drawings.